sigchain API

Code often wants to set a signal handler to clean up temporary files or
other work-in-progress when we die unexpectedly. For multiple pieces of
code to do this without conflicting, each piece of code must remember
the old value of the handler and restore it either when:

The work-in-progress is finished, and the handler is no longer
necessary. The handler should revert to the original behavior
(either another handler, SIG_DFL, or SIG_IGN).

The signal is received. We should then do our cleanup, then chain
to the next handler (or die if it is SIG_DFL).

Sigchain is a tiny library for keeping a stack of handlers. Your handler
and installation code should look something like: